The subject invention is in the field of selective irreversible inhibitors of the enzyme monoamine oxidase (hereinafter MAO) and provides the R!(+)enantiomer of N-propargyl-1-aminoindan (also referred to herein as PAI) which is a selective irreversible inhibitor of the B-form of monoamine oxidase enzyme (hereinafter MAO-B). The subject invention also provides pharmaceutical compositions containing R!(+)PAI which are particularly useful for the treatment of Parkinson's disease, a memory disorder, dementia, depression, hyperactive syndrome, an affective illness, a neurodegenerative disease, a neurotoxic injury, brain ischemia, a head trauma injury, a spinal trauma injury, schizophrenia, an attention deficit disorder, multiple sclerosis, and withdrawal symptoms.
Parkinson's disease is widely considered to be the result of degradation of the pre-synaptic dopaminergic neurons in the brain, with a subsequent decrease in the amount of the neurotransmitter dopamine being released. Inadequate dopamine release, therefore, leads to the onset of disturbances of voluntary muscle control, which disturbances are symptomatic of Parkinson's disease.
Various methods of treating Parkinson's disease have been established and are currently in widespread use, including, for example, the administration of L-DOPA together with a decarboxylase inhibitor such as L-carbidopa or benserazide. The decarboxylase inhibitor protects the L-DOPA molecule from peripheral decarboxylation and thus ensures L-DOPA uptake by the remaining dopaminergic neurons in the striatum of the brain. Here, the L-DOPA is converted into dopamine resulting in increased levels of dopamine in these neurons. In response to physiological impulses, these neurons are therefore capable of releasing larger amounts of dopamine at levels which approximate the normal required levels. L-DOPA treatment thus alleviates the symptoms of the disease and contributes to the well-being of the patient.
However, L-DOPA treatment has its drawbacks, the main one being that its effectiveness is optimal only during the first few years of treatment. After this period, the clinical response diminishes and is accompanied by adverse side effects which include dyskinesia, fluctuation in efficacy throughout the day ("on-off effect") and psychiatric symptoms such as confusional states, paranoia, and hallucinations. This decrease in the effect of L-DOPA treatment is attributed to a number of factors, including the natural progression of the disease, alteration in dopamine receptors as a consequence of increased dopamine production or increased levels of dopamine metabolites, and pharmacokinetic problems of L-DOPA absorption (reviewed by Youdim, et al., Progress in Medicinal Chemistry, 21, 138-167 (1984)).
In order to overcome the drawbacks of L-DOPA treatment, various treatments have been devised in which L-DOPA is combined with MAO inhibitors with the aim of reducing the metabolic breakdown of newly formed dopamine.
MAO exists in two forms known as MAO-A and MAO-B which are selective for different substrates and inhibitors. For example, MAO-B more efficiently metabolizes substrates such as 2-phenylethylamine, and is selectively and irreversibly inhibited by (-)deprenyl as described below.
It should be noted, however, that treatments combining L-DOPA with an inhibitor of both MAO-A and MAO-B are undesirable, as they lead to adverse side effects related to an increased level of catecholamines throughout the neuraxis. Furthermore, complete inhibition of MAO is also undesirable as it potentiates the action of sympathomimetic amines such as tyramine, leading to the so-called "cheese effect" (reviewed by Youdim et al., Handbook of Experimental Pharmacology, ed. by Trendelenburg and Weiner, Springer-Verlag, 90, ch. 3 (1988)). As MAO-B was shown to be the predominant form of MAO in the brain, selective inhibitors for this form are thus considered to be a possible tool for achieving a decrease in dopamine breakdown on the one hand, together with a minimization of the systemic effects of total MAO inhibition on the other.
Many inhibitors of MAO are chiral molecules. Although one enantiomer often shows higher relative potency towards MAO-A or MAO-B, it is impossible to predict a priori which configuration will be the more active or the degree of selectivity between them. Several examples of enantiomeric pairs of MAO inhibitors are tabulated in Costert, P., Medicinal Research Reviews, 9, 45 (1989). In general, there is only a small (less than a factor of ten) difference in potency between R and S isomers in a pair.
N-propargyl-1-aminoindan (PAI), as disclosed herein, is unique in regard to the relative potency between its R and S isomers, when compared with other MAO inhibitors, particularly (R)-N-methyl-N-propargyl-1-aminoindan (R-1-MPAI). The R and S enantiomers of PAI show a 7,000-fold difference in potency, whereas the R and S enantiomers of MPAI show only about a ten-fold difference in potency. Accordingly, this 7,000-fold potency difference would certainly not have been expected, and indeed would have been highly surprising to one of ordinary skill in the the art.
Table I lists the IC.sub.50 (mmol/L) of enantiomeric pairs of propargyl amines in a rat brain preparation of MAO. These results show small differences in potency in MAO-B inhibition between the R and S enantiomers. Both enantiomers are selective for MAO-B. For example, in rat liver homogenate, R!(-)deprenyl is only 15 times as potent as the S enantiomer. (B. Hazelhoff, et al., Naunyn-Schmeideberg's Arch. Pharmacol., 330, 50 (1985)). In other pharmacological activity assays, such as for the inhibition of tyramine uptake, deprenyl shows different stereoselectivities. The S form is in certain cases the more potent epimer. (J. Knoll and K. Magyar, Advances in Biochemical Psychopharmacology, 5, 393 (1972)).
N-Methyl-N-propargyl-2-aminotetralin (2-MPAT) is a close structural analogue of deprenyl. The absolute stereo-chemistry of 2-MPAT has not been assigned. However, the (+) isomer is selective for MAO-B and the (-) isomer is selective for MAO-A. The difference in potency between the 2-MPAT enantiomers is less than 5-fold. (B. Hazelhoff, et al., id.). The lack of data in Table I showing clear structure-activity relationships between isolated (+) or (-)2-MPAT makes it impossible to predict the absolute stereochemistry thereof. Likewise, in contrast to expectations based on PAI, it was found that (S)-N-propargyl-1-aminotetralin (1-PAT) is a slightly more potent inhibitor of MAO-B than its (R) enantiomer. In any case, the difference in potency is small.
After extensive computer modeling, Polymeropoulos recently predicted that R-1-MPAI would be more potent than (S) as a MAO-B inhibitor. (E. Polymeropoulos, Inhibitors of Monoamine Oxidase B, I. Szelenyi, ed., Birkhauser Verlag, p. 110 (1993)). However, experiments described below show that R-1-MPAI is only a slightly more potent inhibitor of MAO-B than is S-1-MPAI, but is an even more potent inhibitor of MAO-A. Both the selectivity between MAO-A and -B and the relative potency of the R and S epimers are low. Thus, contrary to expectations in the art, 1-MPAI is useless as a pharmaceutical agent.
The data presented below demonstrate that high selectivity for MAO of one enantiomer versus the other cannot be predicted. The structure of the MAO active site is not well enough understood to permit the prediction of the relative potency or selectivity of any given compound or pair of enantiomers thereof.
TABLE I __________________________________________________________________________ IC.sub.50 (mmol/L) Data for Rat Brain MAO Inhibition by Propargylamines RELATIVE POTENCY INHIBITION +/- COMPOUND REF EPIMER A B A/B A B __________________________________________________________________________ 2-MPAT a + 140 16 8.8 3 0.2 - 46 88 0.5 DEPRENYL a S 3600 16 120 R/S R 450 6 75 80 2.6 1-MPAI b S 70 50 1.4 23 5 R 3 10 0.3 2-PAT c S 3800 50 76 4 0.5 R 900 90 10 1-PAI c S 26000 17000 1.5 356 6800 R 73 2.5 30 __________________________________________________________________________ a. B. Hazelhoff, et al., NaunynSchmeideberg's Arch. Pharmacol., 330, 50 (1985). b. European Patent Application 436,492 A2, published July 10, 1991. c. Present inventors.
One selective MAO-B inhibitor, (-)deprenyl, has been extensively studied and used as a MAO-B inhibitor to augment L-DOPA treatment. This treatment with (-)deprenyl is generally favorable and does not cause the "cheese effect" at doses causing nearly complete inhibition of MAO-B (Elsworth, et al., Psychopharmacology, 57, 33 (1978)). Furthermore, the addition of (-)deprenyl to a combination of L-DOPA and a decarboxylase inhibitor administered to Parkinsons's patients leads to improvements in akinesia and overall functional capacity, as well as the elimination of "on-off" type fluctuations (reviewed by Birkmayer & Riederer in "Parkinson's Disease," Springer-Verlag, pp. 138-149 (1983)). Thus, (-)deprenyl (a) enhances and prolongs the effect of L-DOPA, and (b) does not increase the adverse effects of L-DOPA treatment.
However, (-)deprenyl is not without its own adverse side effects, which include activation of pre-existing gastric ulcers and occasional hypertensive episodes. Furthermore, (-)deprenyl is an amphetamine derivative and is metabolized to amphetamine and methamphetamines, which substances may lead to undesirable side effects such as increased heart rate (Simpson, Biochemical Pharmacology, 27, 1951 (1978); Finberg, et al., in "Monoamine Oxidase Inhibitors--The State of the Art," Youdim and Paykel, eds., Wiley, pp. 31-43 (1981)).
Other compounds have been described that are selective irreversible inhibitors of MAO-B but which are free of the undesirable effects associated with (-)deprenyl. One such compound, namely N-propargyl-1-aminoindan HCl (racemic PAI.HCl), was described in GB 1,003,686 and GB 1,037,014 and U.S. Pat. No. 3,513,244, issued May 19, 1970. Racemic PAI.HCl is a potent, selective, irreversible inhibitor of MAO-B, is not metabolized to amphetamines, and does not give rise to unwanted sympathomimetic effects.
In comparative animal tests, racemic PAI was shown to have considerable advantages over (-)deprenyl. For example, racemic PAI produces no significant tachycardia, does not increase blood pressure (effects produced by doses of 5 mg/kg of (-)deprenyl), and does not lead to contraction of nictitating membrane or to an increase in heart rate at doses of up to 5 mg/kg (effects caused by (-)deprenyl at doses over 0.5 mg/kg). Furthermore, racemic PAI.HCl does not potentiate the cardiovascular effects of tyramine (Finberg, et al., in "Enzymes and Neurotransmitters in Mental Disease," pp. 205-219 (1980), Usdin, et al., Eds., Wiley, New York; Finberg, et al. (1981, in "Monoamine Oxidase Inhibitors--The State of the Art," ibid.; Finberg and Youdim, British Journal Pharmacol., 85, 451 (1985)).
One underlying object of this invention was to separate the racemic PAI compounds and to obtain an enantiomer with MAO-B inhibition activity which would be free of any undesirable side effects that are not a direct consequence of the exclusive inhibition of MAO-B, in the treatment of disorders discussed infra.
Since deprenyl has a similar structure to PAI and it is known that the (-)enantiomer of deprenyl, i.e. (-)deprenyl, is considerably more pharmaceutically active than the (+)enantiomer, the (-)enantiomer of PAI would be expected to be the more active MAO-B inhibitor.
However, contrary to such expectations, upon resolution of the enantiomers, it was found that the (+)PAI enantiomer is in fact the active MAO-B inhibitor while the (-)enantiomer shows extremely low MAO-B inhibitory activity. Furthermore, the (+)PAI enantiomer also has a degree of selectivity for MAO-B inhibition surprisingly higher than that of the corresponding racemic form, and should thus have fewer undesirable side effects in the treatment of the indicated diseases than would the racemic mixture. These findings are based on both in vitro and in vivo experiments as discussed in greater detail infra.
In addition, in whole animal behavioral studies infra relating to neuroprotection or to restoration of normofunction after deliberate lesion to the CNS, the highest active dose of racemic PAI was found to be greater than twice the highest active dose of R!(+)PAI, suggesting interference from S!(-)PAI present in the racemic mixture (see Example 32, Tables 11 and 12).
In certain cases disclosed infra, the in vitro EC.sub.50 of racemic PAI is larger than twice the EC.sub.50 of R!(+)PAI, suggesting interference from S!(-) PAI, as in Table 2, page 46.
Therefore, R!(+)PAI has better potential to be a more effective and safe drug for the treatment of the above-mentioned disorders than does racemic
because it has potentially fewer side effects. Furthermore, in some biological assays S!(-)PAI seems to antagonize the beneficial activity of R!(+)PAI.
It was subsequently shown that (+)PAI has the R absolute configuration. This finding was also surprising based on the expected structural similarity of (+) PAI in analogy with deprenyl and the amphetamines.
The high degree of stereoselectivity of pharmacological activity between R!(+)PAI and the S!(-) enantiomer as discussed hereinbelow is also remarkable. The compound R!(+)PAI is nearly four orders of magnitude more active than the S!(-) enantiomer in MAO-B inhibition. This ratio is significantly higher than that observed between the two deprenyl enantiomers (Knoll and Magyar, Adv. Biochem. Psychopharmacol., 5, 393 (1972); Magyar, et al., Acta Physiol. Acad. Sci. Hung., 32, 377 (1967)). In fact, based on the data in Table I, this high degree of stereo-selectivity by R!(+)PAI over S!(-)PAI in MAO inhibition is unique. Furthermore, in some physiological tests, (+)deprenyl was reported to have activity equal to or even higher than that of the (-)enantiomer (Tekes, et al., Pol. J. Pharmacol. Pharm., 40, 653 (1988)).
MPAI is a more potent inhibitor of MAO activity, but with lower selectivity for MAO-B over A (Tipton, et al., Biochem. Pharmacol., 31, 1250 (1982)). As only a small degree of difference in the relative activities of the two resolved enantiomers was surprisingly observed with MPAI, the remarkable behavior of R!(+)PAI is further emphasized (See Table 1B).
The subject invention also provides methods of using the pharmaceutically active PAI-enantiomer alone (without L-DOPA) for treatment of Parkinson's disease, a memory disorder, dementia, depression, hyperactive syndrome, an affective illness, a neurodegenerative disease, a neurotoxic injury, brain ischemia, a head trauma injury, a spinal trauma injury, schizophrenia, an attention deficit disorder, multiple sclerosis, or withdrawal symptoms (see review by Youdim, et al., in Handbook of Experimental Pharmacology, Trendelenberg and Wiener, eds., 90/I, ch. 3 (1988)).
The subject invention further provides a method of using the pharmaceutically active PAI-enantiomer alone for pretreatment of Parkinson's disease. The subject invention also provides pharmaceutical compositions comprising R!(+)PAI and synergistic agents such as levodopa. The use of such agents has been studied with respect to (-)deprenyl which was shown to be effective when administered alone to early Parkinson's patients, and may also have a synergistic effect in these patients when administered together with .alpha.-tocopherol, a vitamin E derivative (The Parkinson's Study Group, New England J. Med., 321(20), 1364-1371 (1989)).
In addition to its usefulness in treating Parkinson's disease, (-)deprenyl has also been shown to be useful in the treatment of patients with dementia of the Alzheimer type (DAT) (Tariot, et al., Psychopharmacology, 91, 489-495 (1987)), and in the treatment of depression (Mendelewicz and Youdim, Brit. J. Psychiat. 142, 508-511 (1983)). The R!(+)PAI compound of this invention, and particularly the mesylate salt thereof, has been shown to restore memory. R!(+)PAI thus has potential for the treatment of memory disorders and dementia, especially of the Alzheimer's type.
In addition, as disclosed herein, R!(+)PAI has a neuroprotective activity in various in vitro and in vivo models, as well as being effective in models relevant to hyperactive syndrome, stroke, brain trauma, and other neurological disorders, and therefore has potential for treating various neurological disorders, such as hyperactive syndrome, stroke, and brain trauma. R!(+)PAI has significant potential, as disclosed herein, to be a more effective and safer drug for the treatment of the above mentioned disorders than racemic PAI, because R!(+)PAI has potentially fewer side effects than S!(-)PAI. Moreover, in some biological assays, as further disclosed, S!(-)PAI seems to antagonize the beneficial activity of R!(+)PAI.
Finally, the subject invention provides highly stable salts of R!(+)PAI with superior pharmaceutical properties. The mesylate salt is especially stable.